1. Field of the Invention
The present invention relates to protecting inductive load switching transistors from turn-off surge created overvoltage conditions and, more particularly, to achieving protection by means of circuitry the purpose of which is to engage the switching transistor to act towards protecting itself.
2. Description of the Prior Art
When a transistor junction is forced into breakdown by a reverse bias, irreversible damage to the junction will generally occur. If the damage is sustained across a collector-base junction of a bipolar junction transistor, the junction will become leaky. If the damage is sustained across an emitter-base junction, degradation in the gain characteristics of the transistor will occur. The spurious breakdown of a transistor junction during the course of normal operation is thus a hazard against which transistors in general may not be left unprotected.
Typical prior art solutions to protect transistors from surge-created overvoltage conditions utilize shunting arrangements consisting of diodes, Zener diodes, or both. One class of prior art shunting arrangements provide a bypass, or alternative path for a surge current to flow, so constituted as to fully keep a surge created voltage rise from developing across an exposed transistor. Alternative, prior art shunting arrangements permit the development of the surge, but function to clamp the surge created voltage rise to a level that will not exceed the breakdown voltage of an exposed transistor junction. U.S. Pat. No. 4,106,048 issued to H. Khajezadeh on Aug. 8, 1978 discloses a protection arrangement of the first class comprising a diode having a large contact area in shunt with the collector-emitter junction of the protected transistor. In clamp-action arrangements, a Zener diode may typically be involved, where one such arrangement is disclosed in U.S. Pat. No. 4,051,504 issued to J. W. Hile on Sept. 27, 1977. A related patent, U.S. Pat. No. 4,119,440, issued to J. W. Hile on Oct. 10, 1978, discloses the method of making the Zener diode device. Both patents discuss the use of an ion implanted Zener diode with appropriately accurate breakdown voltage, where the Zener diode is connected between the collector and emitter of the protected transistor for the purpose of variously shunting junctions susceptible to damage.
The choice of the protection arrangement in any particular case certainly must depend upon the overall objectives of the circuit. In general, a preferred protection arrangement is one which does not unduly encumber the operation of the protected device, or of the circuit, or both. However, a protection arrangement which also imparts some further operating advantage to the circuit might in some cases also exist. For example, in the commonly used arrangement for protecting relay driver circuits from coil fly-back transients adequate protection to the switching transistor could be offered with a single diode placed in parallel with the coil. However, it is known that placing across the coil a diode and a Zener diode tied together in series, advantageously provides the transistor with protection and further causes the speed of the switching action of the circuit to improve.
Inductive load switching circuits are a class of circuits particularly known for exposing transistor junctions to transient overstressing conditions. Functionally, these circuits switch on and off current activating inductive loads, such as current activating the coil of an electromechanical relay. This action generates inductive surges, and these surges create reverse, transient, overstressing conditions across the junctions of the transistors which are responsible for driving and hence, for switching these loads.